The general concensus is that you won't need a pre. Get a batterybox (with rolloff switch would prolly be best).
The reason the source matters is that with the NJB3 going line in you might need upwards of 50db of gain depending on what you are recording, softish stuff you are going to need a decent mic pre (read decent as 30db+ gain and fairly low noise.) (you can find most of that info in the archive)
As to the source question, it is because you are going to need a batterybox going line in on the NJB3 (and on your mz-n505) IF you went mic in on a MD, your mics would be powered, albeit at a lower voltage...